Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



I have an 8'x10' slab that I am going to build a shed on. The existing slab, while in good shape is not level by slightly over 1". My thought is to cover the entire area w/ new concrete, to get it completely level (without going too much higher over the existing height of the slab). What Sakrete product do you recommend, and approximately how much do you think would be needed? Thanks!
- BobS
Sunday, October 11, 2015 at 11:11 PM

I have concrete footings for a deck post. The footing is about 4" under grade. The cedar post has rotted out at the base and needs replacement. I'd like to bring the concrete up to grade. Can I pour an extension on top of the existing footing, knowing that it will support the weight of the deck?
- M Reid
Saturday, October 10, 2015 at 5:46 PM

Hi, What is the best Sakrete product for vertical application where the old concrete has deteriorated to the point that the corner has now become a rounded edge with deep chunks missing? Thank you!
- Chris
Saturday, October 10, 2015 at 11:15 AM

Ernst, we suggest using Sakrete Top N Bond for this repair. Top N Bond is highly polymer modified for superior adhesion and can be applied from ½” down to a feathered edge. If the new concrete has exceeded the 24 hour time period (Green) you must wait 28 days before applying Top N Bond. If less than 24 hours Top N Bond may be applied once the surface is thumb print hard. Do NOT use a bonding agent with this product.
- Chris Technical Services
Tuesday, October 6, 2015 at 10:14 AM

Hello, I just poured two sets of stairs with a landing half way down, for some reason in the screeding process we ended up with a low spot about 5' long and 20" wide on the landing up against a stair riser. I hope to fill the area with an overlay and cover the rest of the landing with the same material to keep the color the same and put a light broom finish on it. The deepest point of the low spot is about .5". Can you suggest a product that will provide fill the void and allow for a broom finish? Thank you very much.
- Ernst
Monday, October 5, 2015 at 11:41 PM

Henry, you may want to consider renting a concrete grinder to remove the glue from the slab. We suggest using Sakrete Sand Mix modified with Sakrete Bonder and Fortified (as a bonding agent/primer and admixture). You must first properly prep the area by removing any bond breakers such as dirt, dust, debris, loose material, paint, stain, oil, etc. Once the area clean, dry, and solid sound substrate, brush a thin layer of Sakrete Bonder and Fortifier using an inexpensive paint brush to the area. Bonder and Fortifier can be used as an admixture (1:1 totaling the suggested water amount) with Sand Mix. Be sure to honor all expansion joints.
- Chris Technical Services
Monday, October 5, 2015 at 10:14 AM

George, Sakrete High Strength Concrete Mix can be used for the addition around your slab. Excavate around the slab and create forms. Use rebar to mechanically attach the slab to the new concrete.
- Chris Technical Services
Monday, October 5, 2015 at 10:04 AM

I am removing indoor/outdoor carpeting from our patio and want to tile it. We extended our patio years ago, so there are two separate slabs of concrete. Both slabs are in excellent condition. The original slab has settled aprox. 3/4" lower than the second slab, creating a lip. I have two questions. 1) what is the best way to remove the glue under the indoor/outdoor carpet? 2) Once that is done, what would be the best product to use to level out the two slabs? Thanks.
- Henry
Saturday, October 3, 2015 at 2:40 PM

Kevin, we recommend using Sakrete Top ‘N Bond which is a polymer modified resurfacing material that can be used from ½” down to a feathered edge. Sakrete Flo-Coat may be another option which is like Top ‘N Bond in performance but can be used in a flowable consistency and may be easier to use for larger areas.
- Chris Technical Services
Wednesday, September 30, 2015 at 2:05 PM

I have a slab that a double wide sits on, I would lake to make the slab four inched wider on all four sides, how can I do this?
- George
Wednesday, September 30, 2015 at 1:04 PM



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